1. Field of the Invention
The present invention relates to a linear type actuator, in which an output shaft is adapted to move linearly, and more particularly to a linear type actuator which can be manufactured with reduced cost through reduction in material cost and improvement in productivity.
2. Description of the Prior Art
In recent years, a direct drive is required for high performance in various apparatuses, and a linear type actuator has been employed for linear control.
FIG. 1A shows a longitudinal section of a conventional linear type actuator using a PM (permanent Magnet) stepping motor and FIG. 1B shows an enlarged view of a circled portion B of the linear type actuator shown in FIG. 1A.
Referring to FIGS. 1A and 1B, a rotor unit 50 is rotatably arranged inside a stator unit 40, a field magnet 51 is insert molded in the rotor unit 50, and a female screw 52 is formed on the innermost diametral circumferential surface of an insert molded resin portion 56. A male screw 62 is formed on an output shaft 60, and engages with the female screw 52. Due to an antirotation pin 31 preventing the output shaft 60 from rotating, the output shaft 60 is moved in an axial direction by the rotation of the rotor unit 50.
However, the conventional linear type actuator as abovementioned has the following problems:
Since the female screw 52 is required to feature small coefficient of friction, high wear resistance and excellent dimensional stability, insert molding thereof requires a high-quality and therefore expensive resin material pushing up the cost. Furthermore, since its molding conditions include high temperature and high pressure, the workability of insert molding the female screw 52 together with the field magnet 51 is deteriorated and the insert molded product must be rotated to be removed for the purpose of forming the female screw 52. This inevitably complicates a mold and causes an increase in the number of man-hours, which makes it difficult to streamline parts and stabilize the quality of parts as well at the same time.
Further problem is that since the female screw 52 is formed at the time of producing the rotor unit 50, the productivity of the insert molding is poor, thereby increasing the number of man-hours.